Power semiconductor devices, or simply power devices, are usually optimized for their intended purpose. For example, Power-MOSFETs are optimized to be used as switches in Switched Mode Power Supply (SMPS) topologies. In such circuits, the switches are either in ON- or in OFF-state, and operated at high frequency. The main optimization target is typically to minimize losses in the power device by, for example, minimizing the switching time.
Power device may also be used for other purposes such as protective elements for electronic boards or specific delicate components on the electronic board. Large inrush currents and electrical surges may pose a risk for electronic components and can lead to malfunction or destruction of electronic devices. When used as protective element, power devices with reduced or minimized losses are also of interest. In addition to that, the power device may also be provided as dissipative element during switching events in order to protect the electronic board and/or other components on the board.
For example, if an electronic board were to be added to an operating cabinet which is operative, excessive inrush currents may occur as there are usually large capacitors placed at the connectors provided for connecting the electronic board. The capacitors dynamically short the supply voltage. The charging current is only limited by the resistance of the conductor tracks on the board, which may potentially lead to destruction of components or the board. To avoid this problem, the current needs to be limited. Power-MOSFETs may be used as current limiter when operated in the so-called saturation mode: operation at high drain-source voltage UDS and low to moderate drain currents UD. The power device acts like a voltage-dependent resistor in the saturation mode.
The optimization of Power-MOSFETs toward lowest on-state resistance, which is abbreviated as RON, for reducing losses had led to an increase of the transconductance per chip area. The transconductance gm per chip area is a basic parameter of a power device which relates the current drawn from the output of the power device to the voltage appearing across the input of the power device.
On the other hand, a large transconductance may lead to a reduced ruggedness of the power device when operating in saturation mode. The following shall illustrate this. Assuming a small area of the power device is slightly hotter than other areas of the power device. This may occur due to uneven dissipation of generated heat. The locally increased temperature can lead to a local variation of device parameters. The area of increased temperature can carry more current leading to higher thermal losses in that area. As a result, the area having a higher temperature will “attract” even more current from colder areas leading to a potential thermal runaway. This tendency can be expressed by the temperature coefficient ∂ID/∂T which describes the temperature dependency of the drain current. A positive value of this temperature coefficient ∂ID/∂T means that the device operation is potentially instable. For modern common devices, the higher transconductance is roughly proportional to the temperature coefficient ∂ID/∂T in the saturation mode.
The positive temperature coefficient limits the so-called safe operating area, abbreviated as SOA, of the device. The SOA is expressed as a region in a logarithmic ID vs. UDS plot where the device can safely be operated without destruction. Manufactures provide a SOA for each power device to allow the customer to set the operation condition for safely operating the power devices without experiencing malfunction. FIG. 15 shows an example for a typical SOA of a power MOSFET. The safe operating area is limited by a number of lines which are characteristic for the specific Power-MOSFET.
The line 201 is the so-called RDS(on)-limit-line which describes the linear dependency between the source-drain voltage UDS and the drain current ID. The slope of line 201 is defined by the specific on-state resistance RON of the MOSFET at the rated junction temperature and the rated gate voltage specific of the semiconductor device. The horizontal line 202 is the so-called package-limit-line, defined by the maximum current which the external wires or connection of the package, in which the MOSFET is embedded, can carry. For example, bond wires may become too hot for currents exceeding a maximum current. The vertical line 203 is given by the maximum breakdown voltage of the MOSFET. The inclined line 204 is referred to as maximum-power-limit-line and expresses the device's capability to dissipate heat. Line 204 depends, inter alia, on the junction temperature, the duration of the pulse length and on the device package. For example, the maximum rated junction temperature in automotive applications can be in a range of about 150° C. Line 204 represents here an exemplary pulse of 10 ms. The maximum-power-limit-line can be calculated by assuming thermal equilibrium between the generated power Pgenerated and dissipative power Pdissipated.
A further limitation is imposed by the above-mentioned risk of a thermal runaway. This risk is increased for devices having a high transconductance leading to a “kink” in the maximum power limit line as indicated by line 205 which is also referred to as thermal-instability-limit-line. Although the device may be in principle capable of tolerating pulses at higher drain-source voltages, the increased risk of a thermal runaway dictates to limit the maximum power. When comparing line 204 with line 205 it becomes apparent that there is a significant reduction of the total area of the SOA which may have practical implications for operating the power device. Basically, the semiconductor device is considered to be thermally unstable if the generated power rises faster than the power which can be dissipated:
            ∂              P        generated                    ∂      T        >            ∂              P        dissipated                    ∂      T      In this case, the semiconductor device is not in thermal equilibrium and may undergo a thermal runaway.
Attempts have been made to enlarge the SOA. However, there is need for further improvement.